UTC and GMT explained
Every time zone is written as a distance from one reference clock, Coordinated Universal Time.
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What UTC is
Coordinated Universal Time (UTC) is the world’s reference time. It is kept by atomic clocks and adjusted now and then by a leap second so it stays close to the Earth’s rotation. UTC never changes for daylight saving, which is why pilots, servers and scientists use it.
How GMT differs
Greenwich Mean Time (GMT) began as the mean solar time at the Royal Observatory in Greenwich, London. Today it is mostly a time zone name: the United Kingdom and Ireland show GMT in winter, and Iceland and several West African countries use it all year. On the clock, GMT and UTC read the same.
Reading an offset
| Offset | Meaning | When it is 12:00 UTC |
|---|---|---|
| UTC−08:00 | 8 hours behind UTC | 04:00 |
| UTC±00:00 | Same as UTC | 12:00 |
| UTC+01:00 | 1 hour ahead | 13:00 |
| UTC+05:30 | 5 hours 30 minutes ahead | 17:30 |
| UTC+09:00 | 9 hours ahead | 21:00 |
To convert between two places, subtract their offsets. New York in winter (UTC−05:00) and Mumbai (UTC+05:30) are 10 hours 30 minutes apart. The time zone converter does this for any date, daylight saving included.
Writing times without confusion
- Use ISO 8601 with an offset: 2026-11-05T09:00+05:30.
- A trailing Z means UTC: 2026-11-05T03:30Z is the same moment.
- Name the city, not just the abbreviation. IST can mean India, Ireland or Israel; see time zone abbreviations.
Questions
- Is UTC the same as GMT?
- For everyday use, yes: both read the same time. UTC is the modern scientific standard kept by atomic clocks; GMT is a time zone name used by the United Kingdom and some African countries in winter.
- What does UTC+05:30 mean?
- The local clock is 5 hours 30 minutes ahead of UTC. When it is 12:00 UTC, it is 17:30 there. A minus sign means behind UTC.
- What does the Z at the end of a time mean?
- Z stands for UTC, sometimes called Zulu time. 2026-11-05T14:00Z is 14:00 UTC.